Release 10 of Long Term Evolution, “LTE”, by the Third Generation Partnership Project, “3GPP”, introduces relaying as a new feature offering a number of advantages. These advantages include improved coverage for high data rates, rapid temporary network deployment, cell-edge throughput improvements, and/or extension of coverage into new areas. Correspondingly, the 3GPP architecture defines a new node type, referred to as a “Relay Node” or “RN”.
As specified in Release 10, a RN cell appears to user equipment, “UEs”, as a separate cell distinct from the network cell that supports the RN cell. The supporting network cell is referred to as a “donor” cell and the eNB—also known as an eNodeB—of the donor cell is referred to as a donor eNB or DeNB. Each RN cell has its own Physical Cell Id., “PCI”, as defined in LTE Rel-8 and transmits its own synchronization channels, reference symbols, etc. Thus, a UE supported by an RN cell receives scheduling information and HARQ (Hybrid Automatic Repeat-reQuest) feedback and other control signaling directly from the RN. Correspondingly, the UE sends its control channel signaling to the RN. From the perspective of the UE, there is no difference in being served from an RN cell as compared to service from a “standard” eNB cell.
The RN connects to a donor eNB via a wireless interface referred to as the “Un” interface—see 3GPP TS 36.300. In turn, the DeNB provides backhaul transport for the RN and all the UEs connected to the RN. The signaling and the radio protocols used on the Un interface are based on the LTE Rel-8 standard, with only small additions and modifications. Section 4.7 in 3GPP TS 36.300 provides an overview of this approach to relay support.
For further explication, FIG. 1 illustrates a known architecture for an LTE-based wireless communication network 10 that includes a radio access network, “RAN”, 12. In the LTE context, the RAN 12 is referred to as an Evolved Universal Terrestrial Radio Access Network or E-UTRAN. The network 10 also includes an associated core network (CN) 14 that may be referred to as an “EPC” or Evolved Packet Core.
The RAN 12 includes or otherwise supports a RN 16, and multiple eNBs 18. At any given time, one of the eNBs 18 acts as a DeNB with respect to the RN 16 and the CN 14 includes a Mobility Management Entity (MME) 20 and a Serving Gateway (S-GW) 22, among other entities, for supporting control- and user-plane connections, respectively, to the UEs connected through the RAN 12. Note that for simplicity no UEs are shown in FIG. 1.
Various defined interfaces interconnect the illustrated entities, including a Un interface 24 coupling the RN 16 to the DeNB 18, an X2 interface 26 interconnecting the eNBs 18, an S1-MME interface 28 between the eNBs 18 and the MME 20 for control-plane signaling, and an S1-U interface 30 between the eNBs 18 and the S-GW 22 for user-plane signaling. The RN 16 terminates the S1/X2 interfaces 28, 26 in the same way as a normal eNB 18. However, the S1-MME interface 28 is not directly connected to the MME/S-GW 20/22 as for normal eNBs. Instead, for the RN 16, the S1 control messages and data are forwarded between the RN 16 and the S1-MME/U interfaces 28 and 30 associated with the DeNB 18.
FIG. 2 provides further illustration of these known interface connections, and in particular illustrates a UE 40 connected to the RN 16 through a “Uu” interface 42. There is a control-plane, “CP”, connection 44 between the UE 40 and the MME 20. Likewise, the UE 40 has a user-plane, “UP”, connection 46 with the S-GW 22. There is a Generalized Tunneling Protocol (“GTP”) tunnel associated with each UE EPS bearer, spanning from the S-GW 22 associated with the UE 40 to the DeNB 18, which is switched to another GTP tunnel in the DeNB 18, going from the DeNB 18 to the RN 16, according to a one-to-one mapping.
Similarly, the X2 user-plane protocol stacks for supporting the RN 16 during inter-eNB handover from a source DeNB 18 to a target DeNB 18 are proxied via the source DeNB. In particular, there is a GTP forwarding tunnel associated with each UE EPS bearer subject to forwarding, spanning from the source eNB 18 to the target DeNB 18. Such forwarding tunnels are switched to corresponding GTP tunnels in the target DeNB 18, going from the target DeNB 18 to the RN 16, according to a one-to-one mapping. In turn, the user-plane packets are mapped to radio bearers over the Un interface 24. The mapping can be based on the QCI associated with the UE EPS bearer, and UE EPS bearers with similar QoS can be mapped to the same Un radio bearer.
It is challenging to provide high throughput and short handover interruption time for UEs 40 in scenarios where several UEs 40 travel together at high speed. Such group mobility scenarios arise, for example, on trains, buses, and airplanes, where potentially large groups of UEs are moving together and being supported by a mobile RN 16 that is located on the moving vehicle. The likelihood of handover failure increases in such scenarios, because potentially many, nearly simultaneous, handover requests must be signaled for the UEs 40 that are supported by the mobile RN 16.
As shown in FIG. 3, mobile RNs 16 may be installed on top of trains and buses and UEs 40 inside the moving vehicles are connected to these RNs 16 instead of being connected to externally installed eNBs 18 or fixed RNs 48. For example, the diagram depicts an eNB 18 within the RAN 12, where that eNB 18 supports mobile RNs 16 mounted on a bus and a train, respectively. It will be understood that the bus and train move through the cell coverage area of the eNB 18. One also sees that the cell may include one or more static RNs 48, such as might be used to improve cell coverage and/or to provide localized, higher data rate service.
Further, the diagram illustrates an example UE 40, which is supported directly by the eNB 18. However, it will be understood that each mobile RN 16 and/or static RN 18 may support multiple UEs 40, although such UEs 40 are not explicitly shown in the diagram. From the perspective of a UE 40 connected to one of the mobile RNs 16, the serving cell of the UE 40 remains the same, despite the RN 16 moving through multiple cells within the network 10. That is, the RN 16 is handed over from eNB cell to eNB cell in the RAN 12. That is, a mobile RN 16 is handed over from one DeNB 18 to the next as it moves through cells of the network 10, while the UEs 40 moving with the RN 16 remain connected to the RN 16.
Of course, handover of the RN 16 must be properly handled. WO 2011/020432 discloses two example approaches for mobile relay handover. However, these known approaches do not reconcile the potential difficulties associated with relocating the RN 16 and its supported UEs 40 from a packet routing sense, with respect to the rest of the network 10, along with minimizing the required handover signaling and any post-handover signaling burdens imposed on the source DeNB 18.